1. Field of the Invention
This invention pertains to a heat transfer apparatus and process, and in particular, to a vertically oriented shell and tube heat exchanger and a process using a falling film on the exterior surface of the tubes.
2. Description of the Related Art
Vertical, falling film shell and tube heat exchangers have been used, for example, as evaporators and crystallizers in applications for providing potable water from salt water and for concentrating fruit and vegetable juices. In many of the applications for vertical falling film heat exchangers, the falling film is formed on the inside of the tubes. However, there are some applications where the falling film is formed on the outside of the tubes.
U.S. Pat. No. 4,519,448, issued to Allo et al., discloses, for use in concentrating fruit and vegetable juices, a vertical, falling film heat exchanger having a liquid distribution member surrounding each tube. The liquid distribution member has an inverted cone shape and is sealed around the tube. A plurality of holes are provided around a horizontal circumference of the distribution member so that liquid passes through the holes, contacts the exterior surface of the tube and flows as a film down the tube.
Vertical falling film shell and tube heat exchangers are finding application in the Kalina cycle used in the power industry. While the Rankine cycle uses water and steam in a thermodynamic cycle, the Kalina cycle uses a multicomponent fluid, such as a mixture of ammonia and water. In this and many other applications, it is desirable to distribute a liquid to each tube so that a film having a uniform thickness is formed on the exterior surface of each and every tube. However, in many applications the liquid loading to the heat exchanger can be low, which makes it difficult to provide a uniform film for each tube.
The heat exchanger disclosed by Allo et al. is believed to not work very well for a low liquid loading because the open area for liquid flow is relatively large. Further, it is too expensive to make a heat exchanger having an individual liquid distribution member for each tube, where some applications require about 5,000 tubes.